447 research outputs found
Singlet-Triplet Transition Tuned by Asymmetric Gate Voltages in a Quantum Ring
Wavefunction and interaction effects in the addition spectrum of a Coulomb
blockaded many electron quantum ring are investigated as a function of
asymmetrically applied gate voltages and magnetic field. Hartree and exchange
contributions to the interaction are quantitatively evaluated at a crossing
between states extended around the ring and states which are more localized in
one arm of the ring. A gate tunable singlet-triplet transition of the two
uppermost levels of this many electron ring is identified at zero magnetic
field.Comment: 4 page
Kondo Effect in a Many-Electron Quantum Ring
The Kondo effect is investigated in a many-electron quantum ring as a
function of magnetic field. For fields applied perpendicular to the plane of
the ring a modulation of the Kondo effect with the Aharonov-Bohm period is
observed. This effect is discussed in terms of the energy spectrum of the ring
and the parametrically changing tunnel coupling. In addition, we use gate
voltages to modify the ground-state spin of the ring. The observed splitting of
the Kondo-related zero-bias anomaly in this configuration is tuned with an
in-plane magnetic field.Comment: 4 pages, 4 figure
Coherent shuttle of electron-spin states
We demonstrate a coherent spin shuttle through a GaAs/AlGaAs
quadruple-quantum-dot array. Starting with two electrons in a spin-singlet
state in the first dot, we shuttle one electron over to either the second,
third or fourth dot. We observe that the separated spin-singlet evolves
periodically into the spin-triplet and back before it dephases due to
nuclear spin noise. We attribute the time evolution to differences in the local
Zeeman splitting between the respective dots. With the help of numerical
simulations, we analyse and discuss the visibility of the singlet-triplet
oscillations and connect it to the requirements for coherent spin shuttling in
terms of the inter-dot tunnel coupling strength and rise time of the pulses.
The distribution of entangled spin pairs through tunnel coupled structures may
be of great utility for connecting distant qubit registers on a chip.Comment: 21 pages, 10 figure
Imaging the lateral shift of a quantum-point contact using scanning-gate microscopy
We perform scanning-gate microscopy on a quantum-point contact. It is defined
in a high-mobility two-dimensional electron gas of an AlGaAs/GaAs
heterostructure, giving rise to a weak disorder potential. The lever arm of the
scanning tip is significantly smaller than that of the split gates defining the
conducting channel of the quantum-point contact. We are able to observe that
the conducting channel is shifted in real space when asymmetric gate voltages
are applied. The observed shifts are consistent with transport data and
numerical estimations.Comment: 5 pages, 3 figure
A cryogenic amplifier for fast real-time detection of single-electron tunneling
We employ a cryogenic High Electron Mobility Transistor (HEMT) amplifier to
increase the bandwidth of a charge detection setup with a quantum point contact
(QPC) charge sensor. The HEMT is operating at 1K and the circuit has a
bandwidth of 1 MHz. The noise contribution of the HEMT at high frequencies is
only a few times higher than that of the QPC shot noise. We use this setup to
monitor single-electron tunneling to and from an adjacent quantum dot and we
measure fluctuations in the dot occupation as short as 400 nanoseconds, 20
times faster than in previous work.Comment: 4 pages, 3 figure
Transmission Phase Through Two Quantum Dots Embedded in a Four-Terminal Quantum Ring
We use the Aharonov-Bohm effect in a four-terminal ring based on a Ga[Al]As
heterostructure for the measurement of the relative transmission phase. In each
of the two interfering paths we induce a quantum dot. The number of electrons
in the two dots can be controlled independently. The transmission phase is
measured as electrons are added to or taken away from the individual quantum
dots.Comment: 3 pages, 4 figure
Transport properties of quantum dots with hard walls
Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation
with an atomic force microscope. This technique, in combination with top gate
voltages, allows us to generate steep walls at the confining edges and small
lateral depletion lengths. The confinement is characterized by low-temperature
magnetotransport measurements, from which the dots' energy spectrum is
reconstructed. We find that in small dots, the addition spectrum can
qualitatively be described within a Fock-Darwin model. For a quantitative
analysis, however, a hard-wall confinement has to be considered. In large dots,
the energy level spectrum deviates even qualitatively from a Fock-Darwin model.
The maximum wall steepness achieved is of the order of 0.4 meV/nm.Comment: 9 pages, 5 figure
In-plane gate single-electron transistor in Ga[Al]As fabricated by scanning probe lithography
A single-electron transistor has been realized in a Ga[Al]As heterostructure
by oxidizing lines in the GaAs cap layer with an atomic force microscope. The
oxide lines define the boundaries of the quantum dot, the in-plane gate
electrodes, and the contacts of the dot to source and drain. Both the number of
electrons in the dot as well as its coupling to the leads can be tuned with an
additional, homogeneous top gate electrode. Pronounced Coulomb blockade
oscillations are observed as a function of voltages applied to different gates.
We find that, for positive top-gate voltages, the lithographic pattern is
transferred with high accuracy to the electron gas. Furthermore, the dot shape
does not change significantly when in-plane voltages are tuned.Comment: 4 pages, 3 figure
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